1. Field of the Invention
This invention relates to slurries and dispersions useful in making abrasive articles. More specifically, this invention relates to abrasive articles made from slurries and dispersions having viscosity modifying particles therein.
2. Description of Related Art
Three common abrasive articles are coated abrasives, bonded abrasives, and nonwoven abrasives. A coated abrasive comprises a backing onto which abrasive particles are adhered with a binder. The backing may, for example, be selected from paper, cloth, film, vulcanized fiber, and the like, or a combination of one or more of these materials or treated versions thereof. The abrasive particles are typically chosen from flint, garnet, aluminum oxide, alumina zirconia, ceramic aluminum oxide, diamond, silicon carbide, cubic boron nitride, and the like. In bonded abrasives, a slurry is prepared comprising a resin and abrasive particles. When the slurry is placed in a mold, the resin is cured, typically using heat and pressure, holding the abrasive particles together to form a three-dimensional object. Examples of bonded abrasives include grinding wheels, honing sticks, dresser sticks and sharpening sticks. Nonwoven abrasives comprise an open, lofty, three dimensional web of fibers bound together at points where they contact by a binder, which may or may not include abrasive particles. In what may be viewed as a combination of bonded and coated abrasives, slurries as described may be coated onto backings and the resin cured via heat and/or addition polymerization.
In producing the above-mentioned abrasive articles by addition polymerization, polymerization may be initiated in a variety of ways, for example, by thermal decomposition of peroxides or radiation (particle or non-particle), or a combination of the two, depending on the chemistry of the resin. Initiators of the photo and thermal types are common. In the case of initiation by particle radiation, polymerization is typically initiated by irradiation of the binder with an electron beam. The chain carrier in the propagation step may be either ionic or contain a free radical.
Binders used to produce abrasive articles may, and preferably do, contain fillers. Fillers are typically organic or inorganic particulates dispersed within the resin and may modify either the binder precursor or the cured binder's properties, or both, or may simply be used to reduce cost. For example, fillers may operate to inexpensively increase the volume of the binder precursor, thus decreasing cost. Also, fillers often make the cured resin harder or more resistant to changes in humidity (see for example U.S. Pat. No. 2,534,805), more heat resistant, and/or less likely to shrink when cured. The latter is important since shrinkage during cure causes considerable stress, which can lead to premature breakdown of the abrasive product. In some instances fillers may also be used as pigments. Fillers typically have small average particle size, are relatively soft by comparison to abrasive particles, and do not themselves significantly abrade the workpiece. Fillers generally comprise materials which are substantially inert or non-reactive with respect to the workpiece acted upon by the abrasive product. However, "reactive" fillers may be desired for a particular application. A reactive filler interacts with the workpiece in some manner.
While use of fillers may be beneficial in reducing cost and for modification of abrasion properties, originally coatable mixtures of resin, abrasive particles and filler may be difficult to render coatable after having set idle because the filler and/or abrasive particles may settle to the bottom of the container. To avoid disposing of the mixture, the mixture must be agitated to redisperse the abrasive and/or filler particles, which is time consuming and not always successful. Thus, in U.S. Pat. No. 5,014,468, col., 18, lines 48-56, it is disclosed that the slurries used therein are constantly agitated. For these reasons, while the use of fillers may reduce cost, formulators generally have not had the luxury of producing a batch of coatable mixture which would remain coatable for extended periods (i.e., days).
U.S. Pat. No. 4,871,376 (DeWald) describes reducing viscosity of resin/filler dispersions by utilizing a silane coupling agent, but the reference teaches that use of fillers having particle size less than about 2 micrometers is to be avoided due to the increase in viscosity. This evidences the general assumption by artisans of ordinary skill that the addition of small particle size fillers increases the viscosity of slurries and dispersions. Solvents can be added to the mixtures to reduce viscosity; however, most of the added solvents are organic compounds which may make handling and processing of binder precursor mixtures problematic or more expensive. See for example U.S. Pat. No. 5,011,513, where N-methyl pyrrolidone is used to control rheological properties of make and size coating precursor dispersions which include fillers having average particle size of 15 micrometers.
There is thus a need in the abrasives art for binder precursors having reduced viscosity at the time they are to be applied to a backing, and which remain in a coatable form (i.e., with the abrasive and/or filler particles still dispersed therein) for long periods of storage time before they are coated onto backings.
Caul et al., in U.S. Pat. No. 4,588,419, disclose coated abrasives made from a combination of electron curable and heat curable resins. The resins may include calcium carbonate filler and a suspending agent, the preferred suspending agent apparently being fumed silica such as that known under the trade designation "Aerosil 200" (average particle size 12 millimicrometers, surface area 200 m.sup.2 /g). Fumed silica suspending agent is utilized in face fill, back fill, make and size coatings in the Examples of this reference. There is no disclosure of a reduction in viscosity using the disclosed suspending agent, nor an increase in the time that the suspension remains coatable.